For standard browsing, I use Opera, mainly for its standards compliance. It includes built-in features that Firefox needs add-ons like Tab Mix Plus or Adblock Plus to implement. I like that I can assign nicknames to bookmarks and type them when I enter a URL; Firefox has this feature now but requires using the bookmark manager to assign the nicknames. In my experience, Opera loads faster than and uses less memory than other browsers.

For web development, I use Firefox because it has several add-ons for creating and debugging HTML, CSS and Javascript.